We recently moved in to a 20 year old home and a terrible humming noise started occurring two weeks ago in the water pipes. The noise sounds very similar to a fog horn. The noise abates for a while if we turn on another faucet or the sprinklers, only to return at another date. The sellers put in a new pressure release valve (gate valve) to the house and uncapped several sprinklers before we moved in. Is this problem due to air, valves ("chatty valves"), or something else? What should we do? Call a plumber?